1. Field of the Invention
This invention relates to exposure control apparatus for photographic cameras and more particularly to such exposure control apparatus including lag compensation.
2. Discussion Relative to the Prior Art
Many photographic cameras have automatic exposure control systems that terminate an exposure in response to an exposure termination signal generated by a light integrating type exposure control circuit. The exposure may be terminated, for example, by quenching an electronic flash and/or by closing a shutter. In any event, a finite amount of time elapses from the time that the exposure control circuit generates the exposure termination signal to the time that the exposure is actually terminated. This elapsed time is commonly called the "lag time" and is caused by electrical and/or mechanical delays that may be minimized by efficient design of the exposure control system, but not entirely eliminated. The exposure that occurs during the lag time can be a substantial portion of the total exposure. Unless this exposure is effectively accounted for by the exposure control system, an appreciable exposure error may result. The lag problem is particularly troublesome for a photographic process that has a narrow "exposure latitude" such as the instant camera processes that are currently on the market.
One aspect of the lag problem is represented by a shutter closing delay when a picture is taken of a scene illuminated with ambient light, sometimes called an "ambient mode" exposure. In such a case, the shutter closing delay, or shutter lag as it is called, can be adequately compensated for by activating the exposure control circuit a predetermined time prior to the opening of the shutter. This predetermined time is commonly called the "lead time" and is usually chosen to be equal or nearly equal to the shutter lag time. Thus, during an exposure in the ambient mode, the light-responsive exposure control circuit "sees" a quantity of light, prior to the initiation of the actual exposure, that is equal or nearly equal to the quantity of light "seen" by the film during the lag time. Consequently, the lag may be effectively accounted for in the ambient mode by providing a corresponding lead.
If the scene illumination changes during the period of exposure, such as occurs when a scene is illuminated by a flash device, commonly called a "flash mode" exposure, the amount of light seen by the exposure control circuit during the lead time may differ significantly from the amount seen by the film during the lag time. As a result, an appreciable exposure error may result in the flash mode. Therefore, in addition to providing a lead time for ambient exposures, some additional or alternative lag compensation must be provided when a camera is operated in the flash mode. One approach derives the shutter lag compensation from scene brightness information in such a way that the timing of the exposure termination signal is advanced by an amount substantially equal to the lag time. In order to do this, the lag compensation scheme, in effect, continuously predicts when the exposure termination signal will be generated at a time in the future that takes into account the lag time, and then actually generates the exposure termination signal as soon as the prediction is affirmative. Such a system is disclosed in U.S. Pat. No. 3,200,723 for shutter lag compensation, and in U.S. Pat. No. 3,519,879 for electronic flash quench lag compensation. The disclosed systems both use a biasing resistor in the base leg of an R-C timing circuit to continuously bias the light-related charge on an integrating capacitor by an amount proportional to the instantaneous scene brightness. The value of the biasing resistor in relation to the capacitance of the integrating capacitor is chosen to provide the desired compensation. When this arrangement, as shown in U.S. Pat. No. 3,200,723, is used in a camera to compensate for shutter lag in the flash mode, an external mode switch is provided for disabling the effect of the biasing resistor when operating the camera in the ambient mode. Furthermore, successful operation of the system in the flash mode requires that there not be significant ambient illumination during the flash illumination since such ambient illumination would confuse the exposure control system and result in an exposure error. Frequently however, it is desirable to provide flash illumination when there is already significant ambient illumination; such operation of a photographic camera is called exposure in the "fill-flash" mode. The prior art flash compensation apparatus described above will not operate satisfactorily in the fill-flash mode; additionally, the mode switch required to switch between the ambient mode and the flash mode adds complexity and cost to the exposure control system.
For the reasons outlined above, the problem has been to provide an exposure control system with lag compensation that will operate in ambient, flash, and fill-flash modes without the requirement of external mode switches for transferring between the operational modes.